1. Field of the Invention
The present invention relates to a motor control device, and more particularly, to a motor control device installed on an electric power steering device and the like.
2. Description of the Related Art
In recent years, types of control which require information on a steering angle of a vehicle such as an attitude control system for a vehicle using an electronic stability control device (such as ESC), a parking assist system for automatically steering a steering wheel, and the like are increasing in the field of the motor vehicle control. As a result, a steering angle sensor is installed on a vehicle in order to detect the steering angle. The steering sensor is directly attached on a steering shaft.
The steering shaft may be rotated regardless of an on/off state of an ignition switch (hereinafter, referred to as IG switch). Therefore, it is necessary to always detect an angle with respect to a reference point for detecting an absolute position of the steering angle.
As a method of detecting an absolute angle of a plurality of rotations of the steering angle, a method described in Japanese Patent Application Laid-open No. 2006-322794, for example, is proposed. In this method, two MR sensors are used, and an absolute angle is detected based on an output relationship between the two sensors. However, the employment of the two sensors increases the cost. Moreover, a range of detecting a plurality of rotations is mechanically limited. Further, it is necessary to pay attention to precise alignment of reference points of the steering angle sensors when the steering angle sensors are installed.
As a method for solving the problems of Japanese Patent Application Laid-open No. 2006-322794, a method described in Japanese Patent Application Laid-open No. Hei 04-242111 (JP No. 1992-242111 A), for example, is proposed. This method employs a rotary encoder for detecting only a relative position of the steering angle, thereby detecting an absolute position of the steering angle. This method employs the rotary encoder, and detects the absolute position of the steering angle immediately after the IG switch is turned on. Note that, a microcomputer 1 and a microcomputer 2 are provided. The microcomputer 1 detects the steering angle during the time when the IG switch is on. Moreover, a backup power supply and a non-volatile memory are provided in case the IG switch is turned off. The microcomputer 1 stores a neutral point of the steering angle in the non-volatile memory immediately before the IG switch is turned off. The microcomputer 2 continuously detects the relative position of the steering angle during the time when the IG switch is off until the IG switch is turned on again. When the IG switch is turned on, the absolute position of the steering angle is immediately detected from the neutral point stored in the non-volatile memory and the relative position during the time when the IG switch is off.
The electric power steering is provided with a motor for generating a steering assist force on the steering shaft. Particularly if a motor control device employing a brushless motor is used, the motor rotation angle is detected to thereby provide assist control. Motor rotation angle detection means used for the brushless motor control can be used as means for detecting the steering angle information. The steering angle information of a vehicle can be inexpensively acquired in this case compared with the case in which a steering angle sensor is added on the steering shaft. Moreover, the motor rotation angle detection means is integrated into the motor control device, resulting in space saving.
The motor rotation angle detection means of the motor control device can detect the relative position of the motor rotation angle only in a period in which the IG switch is on and the motor control is provided, similarly to in the case of the rotary encoder. However, considering a case in which the motor is rotated by an external force during the time when the IG switch is off, it is necessary to continuously detect the motor rotation angle in the period in which IG switch is off and the motor control is not provided in order to detect the absolute position of the steering angle without an error.
Moreover, it is extremely important to reduce a current consumption in order to prevent a battery from being drained in the period in which the IG switch is off. However, the method according to Japanese Patent Application Laid-open No. Hei 04-242111 has a problem that the reduction in the current consumption is not intended.
Moreover, the steering angle information used for the electric power steering device is intended for assisting the steering of a steering wheel by the user, and it is thus necessary to sufficiently consider the safety. Reliability of calculation means for calculating the motor rotation angle is thus important. However, the method according to Japanese Patent Application Laid-open No. Hei 04-242111 has a problem that a configuration for securing the reliability of the calculation means is not described.